Are you sure they took them? There was a bug that caused whole swathes of guild vault items to just disappear:
If it was guild members (or ex-guild members) who took them, then no, you won’t get anything back from Blizzard. They’ll say that you gave them “permission” to remove those items by giving them access to those tabs in your guild vault.
Ways to mitigate this type of thing in the future - as all guilds are potential targets for this type of thing:
- Try to vet new members and don’t let in those you think seem a bit suss.
- Ensure that the first rank of your guild (ie, the one people join on), has no guild vault access.
- Ensure no-one can withdraw gold, and assign no or a reasonable amount of gold for repairs, that your guild can sustain.
- Ensure that new members have to have been in the guild, active, for a reasonable period of time before promoting them to a level where they get guild vault access.
- Give a small amount of access initially, increasing up the ranks.
- Keep an eye on the guild vault regularly - make sure items are in the correct tabs for rank access, put higher value items in higher rank access only tabs.
People in the earlier ranks in my guild can only withdraw 2-3 items/stacks per day in mainly low value item tabs (professions materials, recipes/patterns, transmogs, battle pets), so I could keep an eye on the logs and see who withdrew what - whether or not they could use said items and if they were returning regularly.
Of course that’s a bit more difficult now as, not only did our guild vaults get decimated by the guild vault bug, but also the guild vault logs are now also broken. There are some addons that can help a little bit, but not much.